Graphene, a single atomic layer of hexagonally packed carbon atoms, has drawn significant attention with its outstanding electrical, mechanical, and chemical properties. Various promising applications based on graphene have been demonstrated, such as in electronics, optoelectronics, and chemical/bio sensing. To further the development of graphene technology, it is desirable to synthesize high quality graphene on a large scale. Since the first mechanical isolation of graphene from graphite crystals, intense efforts have been made to develop methods for graphene synthesis, including reduction of graphene oxide, thermal decomposition of SiC, and transition metal assisted chemical vapor deposition (CVD) processes. In particular, graphene synthesized by CVD on Cu substrates has shown great promise owing to its large size, high quality, and transferability to arbitrary substrates.
So far, CVD graphene films have been polycrystalline, consisting of numerous grain boundaries. Typical known processes of graphene synthesis on Cu start with the nucleation of individual graphene grains randomly distributed across the Cu surface. These grains continue to grow with time and eventually merge together to form a continuous polycrystalline film. Recent results have shown that the individual graphene grains before the formation of the continuous film can be a four-lobed polycrystalline single-layer, hexagonal single crystal single-layer, or hexagonal single crystal few-layer, depending on CVD parameters.
Grain boundaries in graphene have been known to degrade the electrical and mechanical properties of the film. The polycrystalline nature of CVD graphene grown on Cu can be a problem for graphene-based devices, since it is difficult to avoid grain boundaries in the fabricated graphene devices, especially in the case of device arrays and circuits. It is therefore desirable to synthesize either large-scale, high quality single crystal graphene films, or individual single crystal graphene grains in a controllable arrangement. Some recent work has shown low-pressure CVD synthesis of graphene single crystal domains with sizes up to 0.5 mm on Cu foil. But the lack of control in domain distribution may still limit further applications.
Previously we have demonstrated a method to grow single crystal graphene on Cu by CVD from small graphene flakes, and to synthesize arrays of graphene grains using pre-patterned multi-layer graphene seeds. In that case, however, an extra CVD process was first required to obtain a continuous multi-layer graphene film on Cu used for the following lithographic patterning of the growth seeds (multi-layer graphene). This disclosure includes a more effective approach to control nucleation of CVD graphene by locally providing high concentration of carbon. In one embodiment of this disclosure, a solid carbon source of poly(methyl methacrylate) (PMMA) is used for enhancing local nucleation, and spatially ordered arrays of single crystal graphene grains can be synthesized at pre-determined sites (electron beam lithographically patterned arrays of PMMA dots in some embodiments) on the Cu surface. These grains can be transferred to any substrate for further characterization and device fabrication. These methods of controlling the locations of graphene nucleation and the synthesis of single crystal graphene arrays offer a promising route to fabricating graphene-based devices free of grain boundaries and with more reliable performance.
The extraordinary properties and vast potential applications of graphene extensively stimulate the development of graphene synthesis for a graphene film with controllable layers, large size and low defects density. Recently, the synthesis of graphene has seen significant progress on metal and SiC substrates. Graphene can be synthesized on polycrystalline Cu foil by CVD with controllable layers as thin as monolayer and large size (on the order of 30 inches in diameter). However, defects, especially domain boundaries (DBs), can severely negatively affect the electrical and mechanical properties of electronic materials; and graphene DBs, where the defects concentrate, have not been controlled well during graphene synthesis in the prior art. Furthermore, growth of single-crystalline graphene, which has no DBs, has so far been achieved only on single-crystalline metal substrates, which are hardly available in a large scale. The emergence of single-crystal Si famously propelled the development of the silicon semiconductor industry. As a candidate in the post-silicon era, the synthesis of single-crystalline graphene is also expected to change the scenario of graphene's research and applications. The present disclosure includes methods for the growth of single-crystalline graphene on polycrystalline copper substrates, as well as the growth of single-crystalline graphene into an array.